Failed Shock Absorber Caused ATV Rollover?

Metallurgical failure analysis was requested of a failed rear shock absorber bracket from a four-wheel ATV involved in a fatal accident.

The failed shock absorber bracket was manufactured from a cast aluminum alloy. All sections of the failed rear shock absorber bracket were not located. An overall view of a portion of one of the failed shock absorber bracket pieces is shown in Photograph A.

Photograph A: Overall view of portion of failed shock absorber bracket.

The failed shock absorber bracket fracture surface shown in Photograph B contained a region of flat fracture, denoted with arrows. A profile view of the fractured shock absorber bracket, shown in Photograph C, reveals that the casting fracture is very straight and flat. The flatness of the fracture surface is denoted in Photograph C with arrows

Photograph B:  Photo of flat fractured area of shock absorber bracket. Arrows denote area of flat fracture.

Photograph C: Profile view of flat fracture. Arrows denote area of flat fracture.

Scanning electron microscopy (SEM) of the failed casting fracture surface was conducted in the areas denoted in Photograph D. The surface of the casting, at a magnification of 14x, revealed considerable casting shrinkage porosity. The casting porosity is shown in the scanning electron micrograph in Photograph E.

Photograph D: Denoted areas examined by Scanning Electron Microscope (SEM).

Photograph E: Scanning Electron Micrographs at magnification (14X) showing casting porosity.

A close-up view of the areas of casting porosity is shown in Photograph F. The characteristic appearance of shrinkage casting dendrites is clearly evident.

Photograph F: Scanning Electron Micrograph at higher magnification (200X) of aluminum casting porosity.

The fracture surface between the shrinkage porosity exhibited a dimple rupture fracture surface, and is shown in Photograph G. No evidence of metal fatigue (as preliminarily indicated by the flat fracture) was observed in the subject failed shock absorber bracket.

Photograph G: Scanning Electron Micrograph of surface between shrinkage porosity exhibiting a dimple rupture fracture surface (Magnification 500X).

From the above studies, it was evident that this portion of the failed ATV shock absorber bracket was not the cause of the subject accident. The dimple rupture fracture surface was the result of sudden impact, overload failure.

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